How to just disable a fluid domain?
You can only bake a fluid sim if it is the only domain object in the scene.

Multiple fluid sims can exist in the same scene, to bake a fluid sim you have to change the fluid type of the other domains while you bake. After the bake is finished you can change all of the fluid types back to domain and play then at the same time.

Be careful that you set different cache paths for each domain otherwise they will overwrite each other.